This invention relates to a diode array colorimeter instrument for measuring light transmission through a test sample and for measuring the turbidity of the test sample.
Colorimetry and spectrophotometry, analytical techniques based on passing light through a colored solution, are the most widely chemical used analysis method in medicine, chemistry, water quality, and environmental science. Spectrophotometry involves relatively high cost instruments that use a continuously variable wavelength of light of quite narrow bandwidth, while colorimetry involves lower cost instruments with several selectable wavelengths or colors and wider bandwidth. Colored solutions absorb certain wavelengths of light because of electron energy level spacings within the absorbing molecule. Absorption of a specific wavelength of light is related to the concentration of the absorbing molecule. Colorimetric measurements, while generally lacking in flexibility because of wavelength limitations, are often of completely adequate quality for environmental and educational use.
Visible-spectrum spectrophotometers use a white light source and either a prism or diffraction grating to select the wavelength of light to pass through the sample. The prism or diffraction grating requires a relatively expensive calibrated moving mechanical mount, and the small amount of light that reaches the sample often requires sensitive photo detectors, thus placing a relatively high baseline cost on these instruments.
Colorimeters use either a white light source and an optical filter to select wavelength or, in the alternative, use light-emitting-diodes to produce light beams having wavelengths. These are switch selected, and the instrument is operated at only one wavelength.
A diode array spectrophotometer uses white light to illuminate the sample, a prism or diffraction grating to separate the white light into a spectrum after passing through the sample, and then an array of 100 or 256 extremely small photo-diodes, each viewing a different color of wavelength of light. The signals from this diode array are scanned electronically and transmitted to a computer for display and analysis. An example of such a spectrophotometer is shown in U.S. Pat. No. 5,477,326 to Dosmann. The diode array spectrophotometer is the most flexible and most expensive of the spectrophotometric instrument family.
While concentration is measured by determining the absorption of a colored light beam passing through a sample, turbidity is measured by observing light scattered to the side as the light beam passes through the sample. Turbidity is of value in determining the concentration of extremely small colloidal particles suspended in a solution. Historically different instruments have been used for measurement of transmitted and scattered light.